Amplifying system



March 21, 1944. A. PREISMAN AMPLIFYING SYSTEM Filed Dec. 31, 1940 W W U0 Zhwentor Lfiman Albert Pre attorney Patented Mar. 21, 1944 AMPLIFYINGSYSTEM Albert Preisman, Flushing, N. Y., assignor to Radio Corporationof America, a corporation of Delaware Application December 31, 1940,Serial No. 372,462

2 Claims.

This invention relates to amplifying systems, and more particularly toamplifying systems having substantially no phase distortion over a widefrequency band.

In television systems wherein it is necessary to transmit an extremelywide band of frequencies and wherein phase distortion is a seriousproblem, it is necessary that the amplifiers employed be substantiallyfree from phase distortion. It is well recognized that the phaserelation between input and output circuits of various frequencycomponents present in video signals must be maintained because anyrelative phase displacement between frequency components results in aserious loss in definition of the reproduced picture. In thisconnection, it is exceedingly important that the low frequencycomponents have their initial phase relation free from any variation ordeviation since the low frequency components represent backgroundvalues. Any phase displacement between frequency components destroys thesharpness of detail inasmuch as thefrequency components do not coincidein proper spacial relation with the background so that a distorted imageis reproduced.

By properly choosing the constants of adjacent plate circuits and thegrid circuit of an amplifier, it is possible to provide an amplifyingsystem whose low frequency response is substantially free from phasedistortion and there is also an increase in the over-all gain of theamplifier.

Furthermore, non-linear very low frequency phase shift usually causesvariation in shading in the background of a scene, and also tilts thelowfrequency blanking pedestals.

It has previously been proposed to provide an amplifying system with aplurality of stages having their plate circuit constants so chosen withrespect to their grid circuit constants of the following stage thatfreedom from phase distortion has been obtained. In my Patent 2,243,121,issued May 27, 1941, I have taken into account the filter circuit of thepower supply feeding the plate circult of a triode tube, as well as theplate load impedance, and by selecting the grid coupling and the gridcircuit of the following triode amplifier so that it is similar in itsimpedance characteristic to thecombined plate load filter cir-' cult. Bymaking the two networks similar in their impedance characteristic, thephase angle of the 50 According to this invention, the input circuit ofa constant current device such as a pentode tube is designed to have animpedance characteristic similar to the impedance of the output circuitof the previous constant current device.

The primary object of this invention is to provide a new and improvedamplifying system which has substantially uniform amplitude and phasecharacteristics over a wide band of frequencies.

Another object of this invention is to provide an amplifying system inwhich the plate load filter circuit of one tube is made to have similarimpedance characteristics to the grid coupling circuit network of thefollowing tube.

Still another object of this invention is to provide an amplifyingsystem including pentode tubes in which the plate load filter circuit ofone tube is made to have-similar impedance characteristics to the gridcoupling circuit network of a following tube.

Other and incidental objects of the invention will be apparent to thoseskilled in the art from a reading of the following specification and aninspection of the accompanying drawing in which Figure 1 is a circuitdiagram showing one form of this invention, and

Figures 2, 3, 4 and 5 show equivalent circuit diagrams for the purposeof illustrating the development of the proper values of circuit elementsto be used in accordance with this invention.

Referring now to Fig. 1, there is shown a signal input circuit whereby asignal including a plurality of frequency components is applied betweenthe control electrode I and the cathode 3 of a tube ii which may be ofthe pentode type. The output circuit of the tube 5 is connected to theanode l and includes a peaking coil 9 connected in series with a loadresistance H which, in turn, is connected in series with a parallelcircuit including a condenser l3 and a resistance ii. A grid couplingcondenser I1 is shunted by a resistance l9 through which the signal isimpressed upon the control electrode 2| of the tube 23. The inputcircuit to the tube 23 which is connected to the control electrode 2|includes a resistance 25 connected serially with a parallel circuitincluding a resistance 21 and a condenser 29. The output signal issupplied by the tube 23 through its anode 3|. The operation and theselection of proper circuit elements may best be explained by referenceto Figs. 2, 3, 4 and 5.

In Fig. 2 there is shown an equivalent circuit diagram of a typicalpentode amplifying tube stage. Such a typical circuit, with theexception of the high frequency peaking coil, is shown in Fig. 1. Theportion shown in broken lines represents the grid coupling circuit ofthe succeeding stage.

I As set forth in more detail in my Patent 2,243,121, issued May 27,1941, the solid line portion of the circuit may be replaced by anequivalent circuit shown in Fig. 3. The transformation equations are:

R1. Ri= (R..+R.

R8 (R r. R I) If Re is regarded as made up of two resistors Tp and Rs inparallel, we obtain Fig. 4, which can then be re-transformed into Fig.by the transformation formulas:

R R, A+ c where It therefore follows that by transforming the pentodeinto some equivalent triode there has been obtained a circuit containingtwo elements Cb and Re which can be used as a bias circuit for the grid.

Let RB (Fig. 3) be broken up into two parallel resistors 1:) and R0 insuch manner that and Rs=Rz.+.Rr, we obtain It will be noticed that R1.and RF and CF (Fig. 2) are predetermined for the actual pentode by highfrequency and decoupling considerations, respectively.

It will be noticed so that, from Equation 2 there may be found for Fig.5:

from Equation 1 that (q+ n+ r) r/ ri-(H it; q t r. R .+R. 1+?) Usually(R -l-Rb) is fixed by the permissible value of resistance in the gridcircuit, for example, R. Also, usually m is a large number compared tounity. Hence, in Equation 3 we have the approximate value From Equation2 there may be obtained after some algebraic manipulation There is thusestablished that all factors which are necessary for determining Cg,Rpg, Cb and Rb in terms of R1... RF, and Cr, and q. The value of q isquite arbitrary and may be chosen so as to make the grid circuitparameters take on any convenient values, although in all cases Rb-l-Rgwill not exceed R, the permissive value of grid circuit resistance.

The method of obtaining parameters Cb and Re for a pentode tube dependssolely upon the circuit equivalents given in Figs. 2 to 5 and uponEquations 1, 2 and 3. The method that follows Equation 3 is merely anattempt to show how Cb. Rpg, Cb and Rh may be conveniently determined interms of the original pentode tube parameters RL, CF, and RF, and is notnecessary to prove that the complete circuit configuration of Fig. 2 ispossible for a pentode tube.

While several systems for carrying this invention into efiect have beenindicated and described,

, it will be apparent to one skilled in the art that this invention isby no means limited to the particular organizations shown and described,but that many modifications may be made without departing from the scopeof this invention as set forth in the appended claims.

I claim as my invention:

1. An amplifying system comprising a first amplifier tube having anoutput load circuit comprising two series resistances with a condenserconnected in parallel with one of the resistances, a second amplifiertube having an input circuit comprising two series resistances with acondenser connected in parallel with one of the resistances, a couplingcircuit between the output and input circuits comprising a, condenserand a resistance connected in parallel therewith, the value of the oneresistance and parallel condenser in said input circuit and the value ofthe condenser and parallel resistance in said coupling circuit being sochosen in accordance with the value of the one resistance and parallelcondenser in said output circuit that the impedance characteristics ofthe input circuit will be similar to 10 the impedance characteristics01' the output circult.

2. An amplifying system comprising a first amplifier tube having anoutput load circuit including two series resistances with a condenserconll nected in parallel with one of the resistances. a second amplifiertube having an input circuit including two series resistances with acondenser connected in parallel with one of the resistances, a couplingcircuit connected between the output and input circuits including acondenser and a resistance connected in parallel therewith, the valuesor the condensers and the parallel resistances in said input andcoupling circuits being so chosen in accordance with the impedance ofsaid output circuit that substantially uniform amplification and phasecharacteristics will be maintained over a wide band of i'requencies.

ALBERT PREISMAN.

